Adding a Reverse reference of Group relationship is impossible in the content index. This means it isn't possible to filter out content in the search by group.
There are already opened issues in the Group module related to this issue:
https://www.drupal.org/project/group/issues/3479177
💬
How to reference groups from content in a search api index?
Active
https://www.drupal.org/project/group/issues/2993759 →
Active
1.35
General code
I provided a solution in the MR that allows to select "Reverse reference: Group relationship using Content" field for the index.
Based on this value the views can filter Content per group.